Our new results published today innature communications provide the last piece of a long studied puzzle. We finally found the actual physical mechanism linking between atmospheric ionization and the formation of cloud condensation nuclei. Thus, we now understand the complete physical picture linking solar activity and our galactic environment (which govern the flux of cosmic rays ionizing the atmosphere) to climate here on Earth though changes in the cloud characteristics. In short, as small aerosols grow to become cloud condensation nuclei, they grow faster under higher background ionization rates. Consequently, they have a higher chance of surviving the growth without being eaten by larger aerosols. This effect was calculated theoretically and measured in a specially designed experiment conducted at the Danish Space Research Institute at the Danish Technical University, together with our colleagues Martin Andreas Bødker Enghoff and Jacob Svensmark.

Background:

It has long been known that solar variations appear to have a large effect on climate. This was alreadysuggested by William Herschel over 200 years ago. Over the past several decades, more empirical evidence have unequivocally demonstrated the existence of such a link, as exemplified in the examples in the box below.

Box 1: Examples demonstrating the Solar/Climate link

Below are several examples showing that the sun has a large effect on climate. The first example is the beautiful correlation between solar activity (as mirrored in theCarbon 14 extracted from tree rings) and Oxygen 18 to Oxygen 16 isotope ratio is stalagmites in a cave in Oman, measured byNeff et al. (2001). The former is a proxy of solar activity (as the solar wind modulates the flux of cosmic rays reaching the terrestrial atmosphere and producing Carbon 14 through spallation).Oxygen 18 is a well known climate proxy (in this case, of the monsoon rain coming from the Indian ocean).

Figure 1: A correlation between 14C/12C from tree rings (a proxy of solar activity) and 18O/16O from stalagmites in a cave in Oman in the southern Arabian peninsula by Neff et al. (2001) (which is a proxy of the temperature of the Indian ocean). A large correlation is apparent.

The second example, by Bond et al. (2001), shows a clear correlation between solar activity (again as recovered using 14C) and the climate of the North Atlantic, as can be reconstructed from ice rafted debris in cores from the ocean floor.

Figure 2: A correlation between 14C/12C from tree rings (a proxy of solar activity) and the amount of ice rafted debris left on the ocean floor in the Northern Atlantic. Again, a large correlation is apparent.

The third example, on shorter time scales, is the clear correlation between solar activity over the past century, exhibiting the quasi periodic 11 year solar cycle, and the rate of change of the sea level.

Figure 3: The correlation between solar activity (in red) and the sea level rate of change from tide gauges across the globe.

Unlike the previous results which qualitatively demonstrate the existence of a strong solar/climate link, the last correlation, byShaviv (2008), can be used to quantify the link and show that the solar minimum to solar maximum variations in the solar activity translates into a 1 to 1.5 W/m2 change in Earth’s energy budget. A more recent analysis of satellite altimetry data reveals that the correlation continues. In fact, if one removes the linear trend from glacier melting, almost all the sea level change can be attributed to the sun and the el Niño southern oscillation (Howard et al. 2015).

Figure 4: The correlation between the linearly detrended sea level measured using satellite altimetry (blue dots) and a model fit which includes just two components: The sun and el Niño southern oscillation. The excellent fit implies that the two components are by far the dominant source of sea level change on short time scales.

The fact that the ocean sea level changes with solar activity (see Box 1 above) clearly demonstratesthat there is a link between solar activity climate, but it can be used to quantify the solar climate link and show that it is very large. In fact, this “calorimetric” measurement of the solar radiative forcing is about 1 to 1.5 W/m2 over the solar cycle, compared with the 0.1-0.2 W/m2 change expected from just changes in the solar irradiance. This means that a mechanism amplifying solar activity should be operating—the sun has a much larger effect on climate than can be naively expected from just changes in the solar output.

Over the years, a couple of mechanisms were suggested to explain the large solar climate link. However, one particular mechanism has accumulated a significant amount of evidence in its support. The mechanism is that of solar wind modulation of the cosmic rays, which govern the amount of atmospheric ionization, and which in turn affect the formation of cloud condensation nuclei and therefore how much light do the clouds reflect back to space, as we now explain.

Cosmic Rays are high energy particles originating from supernova remnants. These particles diffuse through the Milky Way. When they reach the solar system they can diffuse into the inner parts (where Earth is) but lose some energy along the way as they interact with the solar wind. Here on Earth they are responsible for most of the ionization in the Troposphere (the lower 10-20 km of the atmosphere where most of the “weather” takes place). We now know that this ionization plays a role in the formation of cloud condensation nuclei (CCNs). The latter are small (typically 50nm or larger) aerosols upon which water vapor can condense when saturation (i.e., 100% humidity) is reached in the atmosphere. Since the properties of clouds, such as their lifetime and reflectivity, depends on the number of CCNs, changing the CCNs formation rate will impact Earth’s energy balance.

The full link is therefore as follows: A more active sun implies a lower CR flux reaching Earth and with it, lower ionization. This in turn implies that fewer cloud condensation nuclei are produced such that the clouds that later form live shorter lives and are less white, thereby allowing more solar radiation to pass through and warm our planet.

Figure 5: The link between solar activity and climate: A more active sun reduces the amount of cosmic rays coming from supernovae around us in the galaxy. The cosmic rays are the dominant source of atmospheric ionization. It turns out that these ions play an important role in (a) increasing the nucleation of small condensation nuclei (a few nm) and (b) increasing the growth rate of the condensation nuclei (which is the effect just published). The larger growth rates imply that they are less likely to stick to pre-existing aerosols and thus have a larger chance of reaching the sizes of cloud condensation nuclei (CCNs, typically > 50 nm in diameter). Thus, a more active sun decreases the formation of CCNs, making the clouds less white, reflecting less sunlight and therefore warming Earth.

Until today we had just empirical results which demonstrate that this link is indeed taking place. The main results are summarized in Box 2 below. In particular, we have seen correlations between solar activity and cloud cover variations, as well as between cosmic ray flux variations arising from changes in our galactic environment and long term climate change using geological data.

Box 2: Examples showing the cosmic ray climate link

The first empirical evidence linking solar activity with cloud cover was the correlation between solar activity (as proxied by the cosmic rays) and changing cloud cover (Svensmark & Friis-Christensen 1997), in particular, the low altitude cloud cover (seeMarsh & Svensmark 2000). Although later data has cross-satellite calibration problems, the correlation continued. Interestingly, Cosmic rays exhibit an odd/even asymmetry because they are the only solar modulated component that “sees” the fact that subsequent solar cycles have opposite magnetic field polarity. The cloud cover appears to exhibit the same asymmetry.

Later, more evidence for the fact that cosmic rays are not only the proxy of solar activity but are an actual part of the climate mechanism appeared in the form of correlations between cosmic ray flux variations that have nothing to do with solar activity and climate variations. Such variations in the cosmic ray flux exit over geological time scales. We showed that one can use Iron meteorites to reconstruct the cosmic ray flux variations over the past billion years. These variations exhibit seven increases due to passages through the galactic spiral arms, on one hand, but appear to correlate with the appearance of ice age epochs on Earth, on the other (Shaviv, 2002, Shaviv 2003, Shaviv & Veizer 2003). Clearly, cosmic ray flux variations that are independent of solar activity appear to have a large effect on climate as well.

The first suggestion for an actual physical mechanism was that ions increase the nucleation of small (2-3 nm sized) aerosols called condensation nuclei (CNs). The idea is that small clusters of sulfuric acid and water (the main building blocks of small aerosols) are much more stable if they are charged. That is, the charge allows the aerosols to grow from a very small (few molecule) cluster to a small stable CN without breaking apart in the process. This effect was first seen in our lab (Svensmark 2006). The effect was seen again in the CLOUD experiment running at CERN (Kirkby 2011). Later experiments have shown that ions accelerate also other nucleation routes in which the small clusters are stabilized by a third molecule (such as Ammonia). That is, ions play a dominant role in accelerating almost all nucleation routes (as long as the total nucleation rate is lower than the ion formation rate).

Figure 7: The Ion induced nucleation effect measured in the lab. Left: The first demonstration in our SKY experiment showing that increased ionization increases the nucleation of small aerosols (typically 3 nm in size). Right: Corroboration of the results in the CLOUD experiment at CERN.

In the meantime, a number of research groups aimed at testing the idea that cosmic ray ionization could help the formation of cloud condensation nuclei (CCN). This was done by using large global circulation models coupled with aerosol physics. The idea was to see if an added number of small aerosols would grow into more CCNs. All of the numerical models gave the result that the small aerosols were lost before they could become large enough, leading to the conclusion that the effect of cosmic rays on the number of CCN over a solar cycle was insignificant (e.g., Pirece and Adams 2009). This could also be explained analytically (Smith et al. 2016). It was therefore proclaimed that the theory was dead.

Given the empirical evidence, it was clear to us that a link must be present, even if the ion induced nucleation mechanism itself is insufficient to explain the link. Thus, our response was to address the same question without using models but instead to test it experimentally. Therefore, in 2012 we tested if small nucleated aerosols could grow into CCN in our laboratory and discovered that without ions present, the response to increased nucleation was severely damped, just like the above-mentioned models; however with ions present, all the extra nucleated particles grew to CCN sizes, in contrast to the numerical model results (Svensmark et al. 2013). So, experiments contradicted the models. The logical conclusion was that some unknown ion-mechanism is operating, helping the growth.

Figure 8: Left: When injecting small aerosols, the relative increase decreases with aerosol size because as aerosols grow they tend to coagulate with larger aerosols. Right: However, when increasing the ionization in the chamber, not only are more aerosols nucleated, the relative increase survives to larger sizes implying that some mechanism is increasing the survivability of the aerosols as they grow.

Following the experimental results showing that increased ionization does indeed increase the number of large CCNs, the natural question to ask was whether these results were caused by the particular experimental conditions—perhaps this mechanism does not work in the real atmosphere. It is therefore fortunate that our Sun carries out natural experiments with the whole Earth.

On rare occasions, “explosions” on the Sun called coronal mass ejections result in a plasma cloud passing the Earth, with the effect that the cosmic rays flux decreases suddenly and remains low for about a week. Such events, with a significant reduction in the cosmic ray flux, are called Forbush decreases, and are ideal to test the link between cosmic rays and clouds. Finding the strongest Forbush decreases and using three independent cloud satellite datasets and one dataset for aerosols, we clearly found a response to Forbush decreases. These results validated the whole chain from solar activity, to cosmic rays, to aerosols (CCN), and finally to clouds, in Earth’s atmosphere (Svensmark et al 2009, Svensmark et al. 2016).

Figure 9: The average effect of the 5 strongest Forbush decreases in the 1987-2007 period on cloud properties. Plotted in red is the reduction in the cosmic ray flux following “gusts” in the solar wind (from Coronal Mass Ejections). In black we see the reduction in aerosols over the oceans and three different cloud parameters from three different datasets (Svensmark et al 2009). These results provide an in situ demonstration of the effect of cosmic rays on aerosols and cloud properties.

With the accumulating empirical and experimental evidence, it was clear that atmospheric ionization is playing a role in the generation of the aerosols needed for cloud formation, however, the exact mechanism proved to be elusive. For this reason, we decided to setup another laboratory experiment mimicking conditions found in the real atmosphere and study how atmospheric ions may be affecting the production of CCNs. This also led us to look for alternative mechanisms which will increase the survivability of the CNs as they growth. Indeed, after several years of research, one was found.

The discovery

A little more than 2 years ago, we made the realization that charge will play a role in accelerating the growth rate of small aerosols. When more ions are present in the atmosphere, more of them end up sitting on sulfuric acid clusters of a few molecules. Moreover, the charge makes the sulfuric acid clusters stick to the growing aerosols much faster, as we explain in the box below. Since faster growing aerosols have lower chances of coagulating with larger aerosols, more of the growing aerosols can then survive to reach larger sizes.In other words, when the ionization rate is higher, more CCNs can are formed.

Box 3: The physics behind the new mechanism

The physics responsible for the accelerated growth is actually relatively simple. A charged cluster of a few molecules of sulfuric acid and water will induce a polarization on the growing aerosols—charge will move from one side of the aerosol to the other, such that one side will be positively charged, while the other negatively charged (with no net charging of the aerosols). Through interaction with this polarization (called Debye force), the cluster and aerosols are attracted. This means that charged clusters stick onto the aerosols notably faster than neutral clusters. Thus, when more ions are present, aerosols can grow faster, and if so, the probability that they stick onto larger aerosols (and thus lost from the system) is smaller.

Figure 10: A negatively charged cluster induces a polarization of the neutral aerosol and then gets attracted to it (since the pull from the positive side of the aerosol is stronger than the push from the negative side).

After realizing that this effect should be taking place we did two things. First, we calculated how large it should be and found that for the typical conditions present in the pristine air above oceans, in which the typical sulfuric acid density is a few 106 molecules/cm3, the ions accelerate the growth by typically 1 to 4%. However, because the number of aerosols surviving the growth is exponentially small (typically several e-folds), the relative change in the CCN density is a few times larger still (by the number of e-folds in the exponential damping to be precise). Thus, over the solar cycle (which changes the tropospheric ionization by typically 20%), we expect a several percent variation in the CCN density and with it, the cloud properties, as is observed.

The second thing we did was go to the lab and design an experiment in which we can see this effect taking place (and also validate our theoretical calculations). This is not trivial because the effect is larger for lower sulfuric acid levels (as a larger percentage of the molecules would be charged). However we cannot measure at very low sulfuric acid levels because the aerosols then grow very slowly such that they stick to the chamber walls before their growth can be reliably measured. This forced us to measure at high sulfuric acid levels for which the effect is smaller. This posed a formidable technological challenge. To overcome this, we designed an experiment which can keep relatively stable conditions over long periods (up to several weeks at a time) during which we could automatically increase or decrease the ionization rate at the chamber. This allowed us to collect a large amount of data and get high quality signals (e.g., see fig. 11 in the box below).

We found that aerosols indeed grow faster when the ionization rate is higher, totally consistent with the theoretical predictions (as can be seen in fig. 12 in the box below). This allows them to survive the growth period without coagulating with larger aerosols.

Box 4: Sample Results

Although the reader is can read the article online, here are a few sample results.

Figure 11: The growth of aerosols in the experiment. Lower panel: Color coded is the number density of aerosols as a function of time (horizontal axis), and diameter of aerosols in nm (vertical axis). Every 2 hours the γ-ray sources are opened/closed. Thus, part of the growth is with high ionization and part with low such that the growth rates can be compared. Top: Since the differences are not large under the experimental conditions, the ionizing sources can be switched on/off over many cycles to get high quality statistics. The reason that the signal is small in the experiment is because growth in the chamber has to be an order of magnitude faster than in the atmosphere, otherwise the aerosols would stick to the chamber walls. Under the faster growth conditions, the effect is smaller.

Figure 12: Difference in the γ-ray open and closed growth times (from 6 to 12 nm), in 11 runs with different sulfuric acid densities (and therefore growth rates) and different change in ionization. The dashed lines are the theoretical predictions.

So, what do the results imply? Until now we had significant amount of empirical evidence which demonstrated that cosmic rays affect climate, but we didn't have the actual underlying physical mechanism pinned down. Now we have. It means that we not only see the existence of a link, we now understand it. Thus, if the solar activity climate link was until now ignored under the pretext that it cannot be real, this will have to change. But perhaps more interestingly, it also explains how long term variations in our galactic environment end up affecting our climate over geological time scales.

Box 5: Why is the CR/climate link ignored?

Given all the empirical evidence that has accumulated until now, the climate community should have considered it seriously, and even if the actual mechanism was until now missing, the empirical evidence showing and quantifying the solar climate link shouldn’t have been ignored by most of the community.

The reason is actually very simple and lays in the implication of the link. If the sun has a large effect on climate, then its increase activity over the 20th century should have contributed at least some of the global warming. In fact, the calorimetric sea level based measurements imply that a bit more than half of the 20th century warming should be attributed to the sun. If so, the role that humans have had is diminished. In fact, when one considers the role that the sun has had over the 20th century, one finds that a) the temperature variations can actually be much better explained (with a smaller residual) and the required climate sensitivity is on the low side (about 1 to 1.5°C increase per CO2 doubling, compared with the canonical range of 1.5 to 4.5°C advocated by the IPCC, see Ziskin & Shaviv, 2012). The low climate sensitivity implies that the same emission scenarios will give rise to more modest temperature increases over the 21st century. These good news imply that we are not in as dire a situations as we often hear. But many do not like hearing this.

Now that the mechanism is actually known, there should be no excuse in ignoring it any further, but given the above implications, it would most likely still be ignored.